The invention relates to a method at the end of a drying section of a paper machine.
In the drying section of a paper machine, drying cylinders are generally placed in two horizontal rows of cylinders, whereby the first drying cylinders are situated substantially higher than the second drying cylinders. In this kind of an arrangement, the web to be dried runs alternately between the drying cylinders of the cylinder rows, the web being supported by the support wire of the cylinder row on the drying cylinders. The main purpose of the support wire arrangement is to press the web into a drying contact with the surfaces of the cylinders and to support the web when running on the surface of the drying cylinders. Between the cylinders of different cylinder rows, the web runs free. Also, Finnish Patent 82850 discloses a multi-cylinder dryer placed after a paper coating machine, wherein a dryer wire and a coated web run first through cylinders arranged in a vertical row and subsequently through cylinders arranged in horizontal rows.
After drying, the web is transferred to the calender section of the paper machine for final treatment of the web. In the machine type that this invention relates to, the calender rolls are placed in the calender section in a vertical row as a so-called roll stack, and the web runs in the calender section from the top downwards; consequently, the web must be brought from the drying section, where it is close to the floor level of the paper machine, to the height of the top of the calender section which is situated considerably higher.
For this purpose, a transfer arrangement is generally provided between the drying section and the calender section, comprising one or several transfer rolls for guiding the run of the web. The transfer rolls are placed one above the other to guide the web along a vertical route, whereby the uppermost transfer roll is approximately at the height of the two uppermost calender rolls of the calender section.
Passing the web intact in the transfer arrangement primarily requires the supporting of the web by transfer rolls arranged at sufficiently short intervals, whereby so-called flutter of the web is avoided. Consequently, the transfer arrangement forms a distinct part of the paper machine, requiring a space of its own in the paper machine. In spite of a number of transfer rolls, the distances between the rolls remain long enough to complicate the run of the web, particularly with light paper qualities. Moreover, threading requires special measures for the same reason.
The purpose of the present invention is to present a novel method for bringing the web from the drying section to the calender section and simultaneously to improve the prior art in the field.
Using the method of the invention it makes it possible to bring the web from the drying section to the calendering section with a separate transfer arrangement comprising several transfer rolls.
In the method, the web is guided by one or several drying cylinders which are located substantially higher than the drying cylinders situated in the upper horizontal row of cylinders at the end of the drying section, whereby the web is guided closer to the uppermost calender rolls while it is still being carried and dried by the drying cylinders.
The method of the invention has particularly two significant advantages. When the web is brought at the end of the drying section by the drying cylinders in vertical direction, deviating from the general horizontal direction of the drying section, it is possible to reduce substantially the floor surface required by the drying section, when it is simultaneously possible to bring the web towards the upper part of the calender section, i.e., said drying cylinders form simultaneously at least part of the transfer arrangement for guiding the web from the drying section to between the rolls of the calender section. Thus, the length of the drying section can be changed in a way that the placement of the calender in an existing paper machine will not increase the total length of the paper machine or reduce the drying capacity. On the other hand, by using this method, the web can be brought into the calender section more securely than by known methods and with a smaller risk of damaging the web, because the open draw between the cylinders of the drying section and the calender rolls is shortened. These advantages are obtained particularly when the wire run arrangement that makes an essential part of the drying section is extended to cover also the cylinders carrying the web in the vertical direction, whereby the web can be carried in a particularly controlled manner. Also the threading of the web is facilitated for the said reasons, and it is possible at this point to use threading systems of the drying section. Thus, conventional edge strip threading to the calender, performed with ropes or blows, will be rendered unnecessary, or the threading strip, leader, can be cut at an advantageous point of the web first in the immediate vicinity of the first nip of the calender.
The invention relates also to an apparatus for applying the method. The apparatus comprises at least one drying cylinder which is situated higher than the level of the upper cylinder row in the end of the drying section before the calender section.
In the case that several drying cylinders are placed higher than the upper horizontal cylinder row, they can be placed in two successive vertical cylinder rows in the machine direction, that is, longitudinal direction of the machine, whereby the uppermost drying cylinder of this upwards directed end can be placed approximately at the height of the uppermost calender rolls. A transfer arrangement of rolls is thus at least partly replaced by drying cylinders.
This construction gives naturally also substantial costs savings, because the transfer arrangement with transfer rolls, required by known constructions, is mainly rendered unnecessary. One should note that these advantages are obtained without having to increase the height of the space required by the paper machine from the height required by the calender section as such. Moreover, there is no need to increase the number of drying cylinders, but the upwards directed transfer can be implemented by using those cylinders of the drying section, with the respective drying capacity, that would normally be located in horizontal rows, i.e., the end of the drying section is, in a way, xe2x80x9clifted upxe2x80x9d and, as a result, the drying section becomes shorter in the machine direction. The placement of the drying cylinders in vertical cylinder rows, their drive and their heating, or the introduction of suction into them can be accomplished also with relatively simple constructions previously used in drying sections.